1. Field of the Invention
This invention relates in general to integrated circuits and in particular to level shifters for integrated circuits
2. Description of the Related Art
Level shifters are utilized in integrated circuits for changing the voltage of a signal from a first voltage to a second voltage.
FIG. 1 is a circuit diagram of a prior art level shifter. A signal applied to input 112 may range from 0 volts to the voltage of a first supply voltage (VDD1). The shifted signal provided at output 114 in response ranges from 0V to the voltage of a second voltage supply (VDD2). Level shifter 101 includes a pair of cross coupled inverters with P-channel transistor 105 and N-channel transistor 107 making up inverter 104 and P-channel transistor 109 and N-channel transistor 111 making up inverter 106. Level shifter 101 also includes an N-channel transistor 113 having a gate connected to input 112 and a second N-channel transistor 115 having a gate coupled to input 112 via an inverter that includes P-channel transistor 117 and N-channel transistor 118.
When input 112 is at VDD1, transistors 107, 109, and 113 are conductive and transistors 105, 111, and 115 are non conductive to pull output 114 to VDD2. When input 112 is at 0V, transistors 107, 109, and 113 are non conductive and transistors 105, 111, and 115 are conductive to pull output 114 to 0V.
During the power up of a circuit incorporating level shifter 101, the output of the voltage regulator supplying supply voltage VDD2 may reach VDD2 faster than the output of the voltage regulator supplying VDD1 reaches VDD1. Because inverters 104 and 106 have the same threshold voltage, the voltage of output 114 can not be relied upon to be in the same voltage state (a known state) (either a high voltage state or a low voltage state) during each power up.
What is needed is an improved level shifter.